Conventionally, as one example of internal combustion engines of a saddle riding vehicle, including a decompression device of a valve train or valve operating system, there has been known an internal combustion engine disclosed in Patent Document 1 below. In this internal combustion engine, a decompression shaft or operating shaft which is rotated under a centrifugal force of a decompression weight of the decompression device is fitted in a decompression shaft hole formed in an exhaust camshaft of the valve train. The decompression shaft hole is positioned on an axis of the exhaust camshaft. In such an internal combustion engine, if it is attempted to form an oil supply passage in the exhaust camshaft for supplying oil to the cam surfaces of intake/exhaust camshafts and to journal portions thereof, it is difficult to additionally form such an oil supply passage, since the decompression shaft is positioned on the axis of the exhaust camshaft. Therefore, a portion of the decompression shaft hole must be used as an oil supply passage for supplying oil to the respective portions of the valve train, and hence it is difficult to supply a sufficient amount of oil to the respective portions of the valve train.